Process of treating magnesia-containing limestones



April 9, 1940.

w. YOUNG 2.196.945

PROCESS OF TREATING HAGNESIA-CONTAINING LIMESTOE ES Filed Feb. 11, 19582 Sheets-Sheet 2 DOLOMITIC ROCK COAL. FLOW SHEET MAKEUP m DOLOMITIC LIMESTEAM WATER (NO. 19"r WATER (2) PROCESS WATER (3) CLASSI PIER. msoLuBLEWASTE (HOOMQsh) (4)A61TAToR,

SGRUBBER, (5)DlS5O-L VE P.S I

FLIOCZCULANT RETURN 50H D5 (MW 32 (Thic kangns) Mm OVEFLLDGW CO FROM KLNMagmslum Hydroxide and (W Undissolved (lalciinm Hydroxide C alci um{-lydmxiidg) (l8)FlLTER, i MAGNESIASLUDGE. HLTRATE (7) PRIMARYCARBONATOR.

- I =F-LoccuLAN-r EL MQ'LL (8) THICKENER CACO SLUDGE.

SECON DAEY CARBONATOR.

. UNDERFLOW /=ERFLOWHr--) (Calcium Carbonzde) (Progess H O) FILTER. I

FILTRATE'. V

DRYER. AND CALCANEK C00 -ER.

CLASSlFlER pH 8 T=mss THAN 60C AGITATORJ Q S FlLTER. m. DRYEK,

PULVERIZER.

High Grade, CBC/O Fl LTRATEW attorneys Znwentor;

Patented Apr. 9, 1940 UNITED STATES PATENT OFFICE PROCESS OF TREATINGMAGNESIA- CONTAINING LIMESTONES William J. Young, Charles Town, W. Va.,assignor to The Standard Lime and Stone Company, Baltimore, Md., acorporation of Maryland Application February 11, 1938, Serial No.190,128

' 28 Claims.

first be recited, with certain emendations, andthen a modification ofsaid process will be given, which modified process has been found toproduce more highly satisfactory results in a shorter period of time,while still utilizing in the main the steps of the-process of theaforesaid application, Serial No. 133,286, and allowing the employmentof smaller apparatus elements.

Methods have been proposed to produce from magnesian limestones, such asdolomite, a high magnesia product. In the main, these methods aredirected to the extraction of magnesia from the dolomite, or theelimination of calcium oxide to produce a product high in magnesia. Themethods, for the most part, depend on differences in the chemicalbehavior of various compounds of magnesium and calcium. However, none ofthe rior methods have proved sufilciently economical to permit theirusefor the manufacture of a high magnesia base. for use in the manufactureof refractories. The operating costs of the prior art methods have beentoo high to produce a 0 magnesia product that could compete with deadburned magnesite for use as a refractory. The

only method which has been successfully worked out and applied on acommercial scale is the Pattinson method and its modifications, whichresults in a very pure basic magnesium carbonate used for insulatingpurposes, and for manufac ture of magnesia for the chemical andpharmaceutical industries, but too expensive to use in refractories. Y

-The present invention is based on the great difference in solubility ofcalcium hydroxide and magnesium hydroxide in water, calcium hydroxidebeing of the order of fifty times more soluble in water than magnesiumhydroxide. Although 45 it has been previously proposedto utilize thisdiilerence in solubility as a basis for processes for the production ofa high magnesia product from dolomite, none of these processes have beenpracticable nor have any been put into operation because of a lack ofunderstanding of certain critical conditions controlling the solution ofthe lime and because of the inability to separate and recover the solidsfrom the very dilute suspensions obtained except by methods costs.

enta i g prohibitive The present invention provides an efllcient methodof treating magnesium-containing limestones, and particularly calcineddolomite,.for the separation of the calcium from the magnesium contentsthereof based upon the relative solubil- 5 ity of calcium hydroxide andmagnesium hydroxide in water.

It provides for reducing the volume of fresh water ordinarily requiredby removing dissolved calcium hydroxide from the water as calcium l0carbonate and reusing the water for further treatment of calcineddolomite. It provides a high magnesia product containing up to 90%magnesia or more and a precipitated calcium carbonate suitable for manyuses. By special treat- 1 ment it produces a portion of thisprecipitated calcium carbonate of such physical properties as to beespecially suited for uses commanding especially high prices such aspaper coating and filler. The present invention further provides forcarrying out the process and steps described in a manner and underconditions such that the process is of proven practicable operation andobvious commercial application, in distinction to other proposed similarprocesses, none of which have ever been proven practical or 'ofcommercial value.

In accordance with the present invention, the limestone is calcined and.slaked, preferably to a milk of lime. The solid particles andaggregates present in the milk'of lime are reduced to a finely dispersedstate, wherebythe calcium content is made more completely amenable tosolution, and this may be accomplished 'by violent agitation of the milkof lime. However, preferably, the dispersion of the particles'of themilk of lime is effected by a combined classifying and agitationtreatment. 'The classification treatment, such as may bedesirablyeffected in a Dorr classifier, removes undesirable and contaminatingmaterials, such as overburned and underburned -lime, sand, stone, andother foreign materials and impurities, and also lumps andagglomerations of the hydrated material that cannot be readilydispersed, and the presence of which reduces the magnesia content of thefinal magnesia product. It may be pointed out that the material to beclassified is a very dilutesuspension of the character hereinafter setforth. Preferably, the classification is effected so as to removeparticles larger than about 100 mesh.

The thoroughly dispersed and classified milk of lime is subjected to theaction of an aqueous dissolving agent with violent agitation, the agentpreferably having a pH within the range of approximately 9.6 to 9.8, toproduce a dilute solution containing dissolved calcium hydroxide andundissolved calcium and magnesium hydroxides in suspension. Theseundissolved hydroxides are separated and recovered. The substantiallyclear solution separated from the solids is then subjected to treatmentto convert at least a portion of the calcium content to calciumcarbonate, and this solid material is separated and recovered. It may bepointed out that while the preferred procedure, for reasons hereinpointed out, is to effect carbonation in two steps, in some cases thecarbonation may be effected in one step, and substantially all of thesolution carbonated at one time. The solution separated from the partialcarbonation may again be treated to convert nearly all the remainingcalcium content to calcium carbonate. These solids are separated andrecovered.

It is desired to point out that the calcium and magnesium componentssuspended in the aqueous solution, as above set forth, are subjected toan aqueous dissolving operation to produce a magnesium-containingproduct having present a cal-. cium component, and that the amount ofcalcium component present is regulated by controlling the amount ofdissolving solution used and the time of solution, the less themagnesium content of the final product and higher the lime content ofthe final product; the less the volume of water used and the solutiontime.

It has further been discovered that an efllcient solution of the limemay be obtained only if the magnesium-containing limestone or dolomitebe calcined under closely controlled conditions of burning, such thatthe calcium carbonate is completely calcined and yet not overburned. Ifthe calcination is carried out at a temperature considerably above thetemperature necessary to dissociation of the carbon dioxide from theoxides (which is approximately 750 0.), then the solubility of theresulting lime is too low for efflcient carryingout of the presentprocess. Preferably, the lime is burned under conditions to produce whatis known as soft burned lime", said conditions being well known in theart.

Producing soft burned lime and treating such lime while still hot entersdirectly into the commercial success of the process. Seemingly, this isdue to the fact that the internal heat of the lime is conducive toextremely rapid and violent slaking, whereby the lime particles areforced asunder into a finely divided condition, thus furthering thesubsequent contact between the water andthe finely divided particles ofthe calcined lime.

Heat alone is not thus effective if the lime is allowed to cool and isafterwards reheated, apparently because such action introduces carbondioxide from the atmosphere whereby the effect of rapid slaking, abovedescribed, is partially defeated.

It is recognized that the solubility of the resulting slaked lime isgreater if the lime is slaked before it has completely cooled. Asillustrative thereof, it may be stated that the lime may be slaked afterburning when the temperature thereof falls within a range of the orderof 149 C. to 177 C.

It has been ascertained that the solubility of the lime is greater ifthe water used in slaking is heated to a temperature above 50 C.Preferably, the temperature will be carried as high as is possible shortof producing too much steam when the slaking operation takes place.

Further, the solubility of the lime in water is greater if the quantityof water or any other aqueous medium used for slaking the calcineddolomite, or other lime base, is in excess of the chemical requirementsnecessary to form the hydrates; or, stated differently, if the amount ofwater used is'sufllcient to produce a milk of lime.

It is desired to point out that the present method is a continuous one.The thoroughly dispersed and classified milk of lime is subjected tosolution in dissolvers, which are equipped with agitators to produceviolent agitation, in which milk of lime and process water are fedcontinuously. The dissolving operation is best carried out at lowtemperatures, as of the order of 20 to 25 C., which is representative ofaverage atmospheric temperature. While higher temperatures 9 may beused, in general, it is desirable that the temperature of the dissolvedsolution be not above atmospheric temperature. It may be pointed outthat refrigeration of the water increases the solubility of the lime toacertain extent. However, it may be noted that it is not commerciallyfeasible to refrigerate thousands of gallons of water per minute, and soeven if the initial dissolving step is carried out at a low temperature,varying just above the freezing point of the solution to atmospherictemperature, the advantage of increased solubility would be lost, sinceultimately a certain amount of the calcium would drop out of solution.

It is desirable to carry out the dissolving operation in high speedagitators at a low temperature and with process water having a pHinsuring the maximum solvent action of the water for the I limehydroxide, and this has been found to be within the range of 9.6 to 9.8.As departure from this range is made, the solvent action of the solventwater decreases. Under these controlled conditions, the solution ofcalcium oxide takes place at the rate of 1 part of calcium oxide to 1000to 1250 parts of water in from 2 to 8 minutes.

It may be pointed out that the dissolving operation is carried out in avery short time, and this is in direct contrast to the prior artmethods( Further, preferably, the solution operation and the removal ofsolids is a continuous operation. It is believed that the increasedspeed of solution and removal of calcium oxide from the solid particlesin suspension is accomplished by the continuous exposure of freshsurfaces of the particles of calcium and magnesium hydroxides to thedissolving action of the processwater, this continuous exposure of freshsurfaces being a result of the violent agitation of the suspension. Thisagitation removes from the surface of the particles the film ofinsoluble magnesium hydroxide as fast as the lime or calcium hydroxideis dissolved, and also removes the solution which has dissolved the limeand is in contact with the particles thereof, and replaces this solutionwith process water less saturated with lime and having a more efllcientsolvent action.

In accordance with the present invention, the lime is made moreaccessible to the dissolving action of the water, or other aqueousmedium employed, than has hitherto been recognized as necessary, andthan has been achieved in the methods heretofore pro'wosed anddescribed, said methods being incapable of commercial utilization andoperation. The utilization of the discoveries herein set forth providesfor an efliciency and'rapidity of solution of the calcium or limecontent of the suspension not previously obtainable, and thereby affordsa method of practical value.

The lime solution from the dissolvers with the magnesium hydroxide andsome lime in suspension, passes continuously to separators in which thesolids are collected and removed. These separatorsare really thickenerswhich concentrate a relatively dilute pulp into a thick pulp, andoverflow a substantially clear calcium hydroxide solution. The time maybe decreased by the addition of reagents that cause the solids tofiocculate. agents are glues and starches.

The underflow from the thickener is passed to a filter and a sludge ofmagnesium hydroxide is obtained having, illustratively, the followingcomposition on a calcined basis:

Per cent MgO 90- CaO 9 Impurities 1 If a sludge of lowermagnesia-content and higher lime-content is desired, a less volume ofwater and less solution time is required, there being considerablelatitude in the process as to the amount of lime separated and dissolvedfrom the original slaked calcium and magnesium-containing material.

The overflow from the thickener or from a battery thereof, is asubstantially clear, very dilute solution of calcium hydroxide fromwhich the lime may be recovered by any of the prior art methods,although it is preferred to recover the lime as calcium carbonate. Thecarbon dioxideproduced during the calcination of the dolomite or otherinitial starting material, is preferably cleaned, as for example, in gasscrubbers and introduced, preferably under pressure, into the calciumhydroxide solution, the carbonation being carried out in what is hereintermed a carbonator. This carbonation results in the formation ofsuspended calcium carbonate'in the process water. The suspension ofcalcium carbonate passes from the carbonator to a primary thickenerwhere a separating action takes place to produce calcium carbonateusually containing some calcium hydroxide and an overflow containingsome calcium hydroxide which has not been carbonated. The separation inthe primary thickeners may be greatly accelerated by the addition offlocculating. agents of the character previously described; Therecovered calcium content which may be in the form of calcium carbonatesludge, may be filtered, dried and pulverized.

It has previously been proposed to recover the calcium content of a limesolution as calcium carbonate by passing CO2 gas into solution and touse the recovered calcium carbonate as a whiting substitute. It has beendiscovered, in accordance with the present invention, that the physicalcharacteristics of the precipitated calcium carbonateare dependent onthe conditions under which it is precipitated. It has been ascertainedthat the recovery of the lime content of the solution may be effected sothat a-considerable portion of the lime is recovered as precipitatedcalcium carbonate of superior quality, which is suitable for such usesas a paper filler and a paper coating.

In accordance with the present invention, there is effected a rapid butcarefully controlled primary partial carbonation of the lime solutionwhich overflows from the magnesia separators.

, cium hydroxide.

Examples of suitable flocculating,

In other words, all the lime content is not carbonated, but only aportion thereof.

Preferably, the carbonating agent is introduced in the primarycarbonator in an amount insufllcient to completely carbonate the limesolution. The amount of carbonating agent that is introduced is thatquantity which will produce a calcium carbonate of exceedingly fineand-uniform particle size containing a small proportion of cal- It hasbeen ascertained that the character of the calcium carbonateprecipitated in the primary carbonator depends upon the hydrogen ionconcentration of the solution, and in order to obtain a substantialamount of high grade calcium carbonate, the carbonation operation iscarried out in twosteps, the first step in a primary carbonator and thesecond in bonation in the primary carbonator is carried to a pointbeyond that represented by a pH of 11.5, the precipitated calciumcarbonate will be of a more crystalline and grainy. nature with largercrystals than is desirable inthe finished product having thecharacteristics above referred to.

It may be stated that the individual particles should as a rule besmaller than 1 micron. As the alkaline value of the solution is reduced,the formation of the calcium carbonate crystals is changed in character.The alkaline value of the solution in the primary carbonator may becontrolled by the amount of carbon dioxide gas introduced therein.

When the primary carbonation is carried out and calcium carbonate isprecipitated, a portion at least of the calcium hydroxide is trapped andpasses to the primary thickener andis recovered therefrom in theunderflow product consisting almost entirely of calcium carbonate withsome calcium hydroxide which usually, although not necessarily, may varybetween 1% to 5%. There is thus produced a calcium carbonate ofexceedingly fine and uniform particle size, containing a smallproportion of free calcium hydroxide, which latter has been trapped bythe calcium carbon ate, out of the substantially but. not perfectlyclear solution. This small 'quantityof unconverted hydroxide would bedetrimental to the commercial precipitated high grade calcium 'carbonatefor certain purposes, such as paper filling and coating. The presence ofthe calcium hydroxide demands an extra expenditure of such relativelyexpensive materials as casein, where such calcium carbonate is to beemployed in paper making and finishing.

Therefore, this calcium hydroxide must be'converted, but preferably willnot be converted in be treated with carbon dioxide, or both, to therebycarbonate the calcium hydroxide. This is washed, filtered, dried andpulverized to produce a material suitable for use as a paper filler anda process. The overflow from the primary thickener carrying a portion ofthe lime in solution, passes to the secondary carbonator, where theremaining lime content is converted to carbonate of lime by treatmentwith a suitable treating.

agent. It is desirable to control the pH of the carbonator solutionwithin the range of 9.6 to 9.8. The dilute suspension of calciumcarbonate produced in the secondary carbonator is preferably fiocculatedwith flocculating agents of the character hereinbefore set forth, andthen passed to a secondary thickener in which the fiocculated solidssettle out rapidly. The overflow water from the secondary thickenerwhich. averages a pH of about 9.7, is returned to the dissolvers for usein dissolving lime from new material. A portion of the process water ispreferably admixed with hot make-up water and used in the slaking step.The separated flocculated calcium carbonate is recovered and used forany purpose wherein a pure calcium carbonate finds application.

Hitherto, calcium carbonate for use as a paper filler and a papercoating has been made by precipitating calcium carbonate from solutionby introducing therein carbon dioxide or soda ash, but such solutionshave had comparatively high concentrations of lime in combined solutionand suspension. The present invention distinguishes 1 from such priorprocesses by recovering the cal- Per cent byweight CaCOa "55.20 MgCOa44.06 510: 0.46 FezOa 0.18 A1203 0.10

The dolomite is calcined in the kiln I, the calcining being carried outto ensure complete calcining of the calcium and magnesium carbonateswith a minimum of overburned lime. It is important for the purpose ofthe present invention,

to ensure the maximum solubility of the cal-' cium oxide in the aqueousdissolving agent, such as water. The lime, preferably while still hot,is passed to a slaker 2 where water is added in a suflicient amount toproduce milk of lime. Preferably, the water is heated to a temperatureof about 50 C., or above.

When a dry hydrated material was produced and afterwards mixed withwater, it was found that the subsequent solubility of the calciumhydroxide was seriously retarded. Therefore, it is essential that morewater be used in this step than is necessary merely to convert thecalcined material into hydroxide. In practice, it was found that a ratioof 3 parts of hot water by weight to 1 part of oxide solids wassufiicient and practicable. However, it is recognized that this ratiomay vary somewhat and still come within the spirit of the presentinvention, depending in some measure upon the character of the limebeing treated, as well as other factors.

The milk of. lime passes from the slaker to a classifier 3. For thepurpose of classification, the milk of lime is greatly diluted.Illustratlvely, the milk of lime may be converted from a material havingawater-lime ratio of 3 to 1, to a suspension in which the ratio of waterto calcined dolomite is approximately 16.6 to 1.

The function of the classifier is to remove from the dilute suspensionundesirable larger sized solids or aggregates that would tend to degradethe magnesium content of the high magnesia product because of theinsolubility of these large sized solids. The classifier also functionsto e1iminate particles of inadequately dispersed dolomitic lime.Preferably, the classifier 3 is so adjusted, that particles larger than100 mesh are eliminated. It is obvious that the milk of lime willcontain particles which will pass through 100 mesh, some that will passthrough 200 mesh, and some that will pass through 300 mesh, or greater.In accordance with the present process, it is highly desirable to putthe calcined product in such an initially fine condition as tofacilitate the solution of the calcium hydroxide contained therein.

The classified milk of lime passes from the classifier 3 to an agitatorl, where the milk of lime is agitated to disperse further the particlesof hydroxides of lime and magnesia. These steps of classification and/oragitation are critical features of the present invention, since thesuccessful solution of the lime content of the suspension is dependentupon thorough mechanical dispersion and violent agitation of thehydroxides to thereby make the process practical by controlling andreducing to a minimum the time in which the solution of the calciumcontent of the sus pension may be accomplished, while simultaneouslydissolving and extracting from the suspension a maximum amount of lime.A thorough classification in the classifier 3 results in a removal ofundesirable constituents at a point in the process where their removalnot only increases the efliciency of the subsequent operations, butresults in a magnesium-containing sludge having a higher recoverablemagnesia content than would otherwise be obtained.

The thoroughly dispersed suspension of calcium and magnesium hydroxidespasses from the agitator l to the dissolvers 5, where additional processwater is added to greatly dilute the suspension. Illustratively,additional process water may be added so that the water is present inthe ratio of one (1) part of calcium oxide in solution to 1000 to 1250parts of water. The process is preferably carried out at a temperaturenot greater than ordinary atmospheric temperature, although the liquidmust not freeze. The point is that it is not desired to limit this stepof the process to solution at atmospheric temperature. Not only is itdesirable to have the temperature of the water in the dissolvers as lowas possible, since this functions to increase the solution of thecalcium hydroxide in the water, but it is I is thereby produced a veryhigh grade calcium desirable to control the pH of the water added in thedissolvers to a range between 9.6 to 9.8. The dissolvers are high speedagitators that produce the violent agitation herein described. This stepof the process is a continuous dissolving action which is completed in afew minutes, as for example, in from 2 to 8 minutes. This rapidity ofsolution of calcium hydroxide has not been obtained in any of the priorart processes.

The solution present in the dissolvers 5- is a dilute solution of limewith magnesium hydroxide and undissolved calcium hydroxide insuspension. To indicate the character of the dilute solution, it may bestated that the solution carries approximately of a gram of calciumhydroxide per liter: From the dissolvers 5, the solution passes toseparators orthickeners 6, these thickeners permitting the solids toseparate as an underflow product and yielding as an overflow product asubstantially clear solution of calcium hydroxide. The time ofseparation in the thickener 6 may be reduced by the addition offlocculating agents, as hereinbefore set forth.

The underflow from the thickener 6 is treated in any suitable manner torecover the solid content of the thickened sludge. This sludge containsmagnesium hydroxide, but may be termed a magnesia sludge, since magnesiamay be recovered therefrom by suitable methods. The thickened materialis preferably fed to a rotary vacuum filter l8 which reduces the watercontent of the magnesia sludge.

-The calcined magnesia sludge produced in accordance with the presentinvention, as above set forth, from the dolomite of the composition,herein specifically referred to, has approximately the followingcomposition:

Per cent MgO 90.0 02.0 9.0

SiOz 0.20

A1203 I 0.45 F6203 0.35

The overflow from the separator 6, which is a substantially clearsolution of calcium hydroxide, is passed to a primary carbonator I,where the carbon dioxide is added in the form of scrubbed kiln gases toa point where the pH of the solution is of the order of approximately11.5 and a controllable portion of the lime has been converted tocalcium carbonate.

The carbonation in the primary carbonator l is carried out rapidly. Itmay be stated that the partial carbonation is controlled by the relationof the amount of gas to the amount of liquid, by the time of contactbetween the gas and liquid, and also by variation of the mechanicalagitation whereby the gas is whipped or churned into the liquid as itpasses through the carbonator.

As an example of the carbonation operation, gas containing about 16%carbon dioxide is continuously passed through the carbonator for aperiod of 8 minutes. Theamount of gas passed through is that which willprecipitate approximately 85% of the calcium content of the liquid, ascalcium carbonate.

It has been found that under the conditions above set forth, if morecarbon dioxide gas is passed through the liquid than is necessary toprecipitate 85% of the calcium carbonate, the resulting carbonate A doesnot conform to the characteristics above referred'to. to point out thatby splitting the carbonation,

-or carrying it out in two separate stages, there It is desiredcarbonate A suitable for paper making, as set forth, and a secondarycalcium carbonate B having different characteristics. It is not intendedto be limited by the above example. Under changed conditions, a greateror less proportion of the calcium content of the solution in the primarycarbonator may be converted into calcium carbonate A. Therefore, it isdesired to point out thatthis aspect of the present invention is notstrictly limited to the percentages or the pH set forth. This feature ofthe invention may be broadly stated to reside in splitting thecarbonation and controlling the percentage of carbonation in the primarycarbonator, to produce a calcium carbonate A of the character specified.

The water containing the suspension of calcium carbonate passes from theprimary carbonator l to a primary thickener 8, where the calciumcarbonate is separated. The detention, or time of separation, may bereduced in the primary thickener 8 by the use of flocculating agents ofthe character hereinbefore described.

Two products are recovered from. the primary thickener, namely, anunderflow consisting of the precipitated calcium carbonate whichcontains a small percentage of free lime, as for example, 1% to 5%, andan overflow product which contains a controllable proportion ofunconverted calcium hydroxide.

The underflow product from the thickener 8 containing a small percentageof calcium hydroxide, is passed to a slurry mixer 9 where an agent isadded to convert the calcium hydroxide to calcium carbonate, preferablyan alkali carbonate, such as sodium carbonate.

The discharge from. the slurry mixer 9 is passed to a continuous filterI 0. The valuable content of this filtrate, as for example, sodiumhydroxide produced through the reaction of sodium carbonate with thelime, is treated with an agent adapted to regenerate carbonate for reusein the process. Preferably, this agent is carbon dioxide derived fromthe calcination of the dolomitic lime. Y

The calcium carbonate may be treated in any suitable manner to adapt itfor commercial utilization. Preferably, it 'is dried in steam dryers IIand pulverized in pulverizers l2. It is then packed and shipped asprecipitated calcium carbonate 22. I This product is a high gradeprecipitated carbonate having an exceedingly flne and uniform particlesize suitable for use as a paper filler and for coating purposes and forall other purposes requiring high grade precipitated calcium carbonate.

The overflow from the primary calcium carbonate thickener 8 passes tothe secondary carbonator l3 where most of the calcium hydroxide isconverted to calcium carbonate, the carbonation of the process waterbeing preferably carried to the point where its pH'is in the approximaterange of 9.6 to 9.8. The dilute suspension of calcium carbonate in theprocess water may have a fiocculent introduced therein as at II and thenbe passed to a secondary thickener II where the solids are collectedrapidly and are recovered as ordinary precipitated calcium carbonate.The overflow of the secondary thickener I5 is process water whichpreferably is returned to the beginning of the cycle for reuse as thedilution agent for the dolomitic lime.

While the above example is illustrative of one manner of carrying outthe present invention, it is not desired to be strictly limited thereto.

Furthermore, while in the specific example set forth, a product isproduced which contains approximately 90% magnesia, it is obvious that aproduct of a greater or less percentage of magnesia may be produced,depending on the amount of calcium removed from the raw material.

Any carbonate rock containing calcium carbonate and magnesium carbonatemay be used regardless of whether these ingredients are present in theratio that they are present in a dolomitic lime. The presence ofrelatively large amounts of impurities is also permissible, providedthese impurities may be separated by classification, or provided theseimpurities do not affect the quality of the product for the purpose forwhich it is to be used.

It will be readily appreciated by those skilled in the art that anysuitable form of apparatus elements may be employed to effect thenecessary and desirable agitation of the materials undergoing thetreatment.

It is essential to a full realization of the invention that theadmixture passing to the agitator be thoroughly beaten so that the limeparticles will be wholly suspended or distributed throughout the watercarrying the same and passed in such condition to the dissolver whereinthe violent agitation herein described is maintained. Various types ofcommercial agitators or mixers may, as above indicated, be employed;such, for instance, as those utilizing a high speed impeller coactingwith deflecting blades, the action upon the water and the lime particlesbeing such that no settling of the particles can inhere.

In order to illustrate the difference between normal and violentagitation, it may be stated that normal commercial agitation may bedefined as agitation required ina mixture of a liquid and finely dividedsolid particles to maintain the solid particles in suspension throughoutthe liquid. Such agitation may be carried out in practice by a singleagitator comprising an impeller surrounded by stationary curveddeflecting blades rotating at a peripheral speed of '700 feet perminute.

In order to illustrate the efiect of normal agitation in the dissolvers,it may be stated that in the presence of 1000 parts by weight of waterto 1 part of calcium hydroxide dissolved therein under normal agitation,there is obtained a product showing 70% magnesia on a calcined basis,this corresponding to the underflow from the thickener 6.. Further, thesolution of tlie calcium hydroxide under these circumstances in thedissolver 5 required from 20 to 25 minutes, and with agitation of thischaracter, it is not practical to produce a 90% MgO product on acalcined basis.

By violent agitation is meant an agitation of the order of about four(4) times the magnitude of normal agitation, above defined.

By increasing the agitation in the dissolvers to four (4) times normal,there was obtained a 90% MgO product on a calcined basis with a solutionperiod of from 2 to 8 minutes, using approximately 1250 parts of waterto 1 part of dissolved calcium oxide. This fourfold increase inagitation was obtained by adding the equivalent of two more agitatorsand increasing the speed of each of the impellers of the compositetriplex agitator one-third. In other words, the peripheral speed of eachimpeller was increased to a little over 900 feet per ute.

The above indicates .that the increased agitation may be obtained by theaddition of more impellers in the dissolvers, or by increasing the speedof each impeller, but preferably by a combination of the two.

Thus, by reason of the fact that the particles are violently thrownagainst one another and in intimate admixture with the process water, aswell as impacting the walls of the tank or container and the agitator orheater elements, there obtains an attrition of surfaces of the limeparticles thereby exposing to the action of the water fresh calcinedlime which is more readily dissolved than would otherwise be the case.

As indicated at the outset, the process as above described may beslightly modified to render it more highly efiicient and to reduce thesize and cost of certain of the apparatus elements which are necessarilyemployed in carrying out this modified and improved method. So, too, themodification of the process enables one to speed up the operation.

With those points in view, among others, the following exposition isgiven, reference being had to flow sheet No. 2.

One of the objects of the modified method is to provide means wherebythe suspended insoluble magnesia residue that remains after the lime isdissolved in water may be recovered more rapidly and more eificientlythan heretofore, and hence permit a corresponding reduction in the sizeand cost of the settling apparatus which may be employed.

So, too, the modified process provides a similar method of recoveringthe calcium carbonate precipitated from the solution of lime in waterafter the insoluble magnesia has been separated from it by causing it tosettle more rapidly and more efdciently, and hence permit acorresponding reduction in the size and cost of the settling apparatusthat may be required.

As outlined in connection with the first process herein set forth, thelime solution from the dissolvers, with the magnesium hydroxide and somelime in suspension, passes continuously to separators in which thesolids are collected and removed. These separators are really thickenerswhich concentrate a relatively dilute pulp into a thick pulp, andoverflow a substantially clear calcium hydroxide solution. Due to theslow settling rate of the suspension, this step of the process as aboveset forth, requires large settling equipment and long detention time.When the size of the settling equipment and the detention time are keptwithin practicable limits, an appreciable amount of the magnesia solidsis carried off with the overflowing lime solution, decreasing the amountof magnesia recovered in the magnesia product and increasing themagnesia content of the calcium carbonate recovered from the limesolution.

My original application, Serial No. 133,286, recognized the fact thatthe size of the settling equipment and the detention time may be reducedby the addition of flocculating agents to the feed to the separators.This step may be modified to advantage, whereby there is accomplished amuch greater reduction in the size of the settling equipment and in thedetention time by greatly increasing the rate of overflow of theseparators, and at the same time accomplishing a more completeseparation of the suspended solids from the lime solution. This isbrought about by seeding the feed to the separators with previouslysettled solids, and at the same time adding small amounts offlocculating agents.

Suitable flocculating agents are glues and starches. The previouslysettled solids are preferably the underflow from the seperators.

, As an example of the improvement over the flrst described process inthe magnesia separation step which may be accomplished by the method nowbeing described, the following is given: With the natural settling rateof the suspension in the thickener, an overflow rate of 2 feet per hourwith a detention of 2 hours in the thickener, yielded an overflowcontaining 0.09 gram of suspended solids per liter of overflowing limesolution. With' the addition of 1 pound of flocculating agent in theform of cheap starch to 12,000 gallons of water in the thickener feed.an overflow rate of'5 feet per hour with a detention of 1 hour yieldedan overflow containing 0.08 gram of suspended solids per liter ofoverflowing lime solution,

On seeding the feed to the thickener with 5 parts by weight ofpreviously settled solids to 1 part by weight of solids suspended in thedissolver discharge flowing to the thickener and adding at the same time1 pound of flocculating agent in the form of cheap starch to 12,000 gal-Ionsv o1 water in the thickener feed.'an overflow rate of 10 feet perhour with a detention of half an hour in the thickener, yielded anoverflow containing only 0.04 gram of suspended solids per liter ofoverflowing lime solution.

This improvement in the separation step enables one to reduce the sizeof the settling equip-- ment required to one-flfth and at the same timereduces the amount of suspended magnesia solids carried over in the limesolution more than onehalf as compared with the natural settling rate;

It likewise enables one to reduce the size of the settling equipmentrequired to one-half that required when the flocculating agent alone isused. and at the same time reduces the amount of suspended magnesiasolids carried over in the lime solution one-half. h

In the process set forth in the early part of this specification, and asdepicted in my copending application, Serial No. 133,286, the limesolution overflowing from the magnesia separators is carbonated in twostages, and the calcium carbonate formed in each stage is recovered bysettling in thickeners. The overflow water of the second thickener isprocess water substantially free of lime that is returned to thedissolvers for reuse.

The thickeners concentrate relatively dilute suspensions of calciumcarbonate into a thick pulp and overflow a substantially clear water.

The overflow of the flrst thickener contains a substantial proportion ofthe,lime in solution which is carbonated and then flows to the secondthickener. The second thickener removes this calcium carbonate andoverflows process water substantially free of lime.

Due to the slow settling rates 01'' the minutely divided suspensions ofcalcium carbonate, these two settling steps of the process require largesettling equipment .and long detention time. When the size of thesettling equipment and the detention time are kept within practicablelimits, a small amount of the suspended calcium carbonate solids iscarried off in the process water and is carried back with it to themagnesia thickeners where it is settled with the magnesia underflow ofthe magnesia thickeners and decreases V discloses that the size of thesecond calcium carbonate thickener and the detention time in it may bereduced by the addition of flocculating agents to the feed to thisthickener; An'object of the process now being set forth is to provide amodification of the calcium carbonate recovery step whereby the lime isrecovered from the lime solution overflowing fronr the magnesiaseparators in a single carbonation step, wherein the rate of settling ofthe suspended calcium car-- bonate solids is greatly accelerated. v

This acceleration in the rate of settling of the suspended calciumcarbonate permits greatly increasing the rate of overflow of the processwater from the calcium carbonate thickener and great-.

ly decreasing the detention time, whereby a much greater reduction inthe size of this settling equipment is obtained, and at the same time, amore complete separation of the suspended calcium carbonate from theprocess water is achieved, with a corresponding increase in the magnesiacontent of the magnesia product 1 settled in the magnesia separators.This is accomplished by seeding the feed to the calciumcarbonatethickener with previously settled solids and by a furtheraddition of a flocculating agent to the feed to the thickener. Thepreviously settled solids are the underflow from this calcium carbonatethickener.

An example of the improvement in the calcium carbonate separation stepas set forth in the foregoing portion of this case and as depicted in myapplication, Serial No. 133,286, which is'ac- .complished by the methodof the modified process now being described, is as follows: With thenatural settling rate of the calcium carbonate suspension in thethickener, an overflow rate of foot per hour and a detention time of 6hours in the thickener, yields an overflow of process water containing0.12 gram of calcium carbonate per liter of overflowing process water.The addition of 1 pound of flocculating agent in the form of cheapstarch to 12,000 gallons of water in the thickener feed, with anoverflow rate of 5 feet per hour and a detention of 1 hour, yielded anoverflow of process water containing 0.13 grani of calcium carbonate.per liter of overflowing process water.

On seeding the feed to the thickener with 5 parts by weight ofpreviously settled calcium carbonate to 1 part by weight of suspendedcalcium carbonate in the feed to the thickener, and adding -1 pound offlocculating agent in the form of cheap starch to 12,000 gallons ofwater in the seeded thickener feed, an overflow rate of 10 feet per hourwith a detention of /2 hour is obtained, and at the same time the amountof calcium carbonate remaining in suspension in the process wateroverflowing from the thickener is reduced to .05 gram per liter ofprocess water. This improvement in the calcium carbonate recovery stepreduces the size of the settling equipment required to andat the sametime reduces the amount of suspended calcium carbonate carried by theprocess water into the magnesia separators to less-than one-half ascomparedwith the natural settling rate, and reduces the size of thesettling equipment required to that required when the flocculating agentalone was used and at the same time reduced the amount of sus-- pendedcalcium carbonate carried by the process water into the magnesiaseparators to much less than /z.

The above examples are illustrative of the amounts of recirculatedsolids and the amounts of flocculating agents that will produce theresults described. This invention is not limited however, to the use ofthe ratios of previously settled solids or to the amounts offlocculating agents given in the examples.

Less acceleration of the settling rates over the original settling ratescan be obtained by the addition of less solids and less fiocculatingagents than used in the examples. The. addition of larger amounts ofsolids requires the addition of larger amounts of fiocculating agentswithout corresponding decrease in the rate of settling and in thedetention time required.

The calcium carbonate recovered by the above steps of this invention isordinary precipitated calcium carbonate and does not meet the rigidrequirements of the highest grades of precipitated calcium carbonate.The next step in this invention is to convert this calcium carbonateinto a precipitated calcium carbonate that meets the most rigidrequirements of the highest grades of precipitated calcium carbonate.

The calcium carbonate sludge, being the underfiow of the thickener, iscalcined to produce lime. The lime is slaked to a milk of lime by theaddition of an amount of waterequal to 4 parts of water by weight tol/part of lime by weight. The milk of lime is allowed to cool to atemperature of about 40 C. and diluted by adding an amount of waterequal to that used in slaking. The diluted milk of lime is then screenedor classified to under 100 mesh to remove foreign material and anyuncalcined residues that it may contain.

The classified or screened diluted milk of lime is then carbonated in amechanical agitator.

which keeps it in normal agitation during the carbonation. During thecarbonation step, carbon dioxide, preferably in the form of kiln gasescontaining somewhat less than 10% CO1, is slow- 1y introduced into theagitator. The temperature of the liquid in the agitator is controlled bythe introduction of a volume of air such that the temperature is keptunder 60' C., so that the cal-, cium carbonate particles form as calciteand not as aragonite. The carbonation should be continued until the pHof the liquid is below 8. This I will ordinarily require at least 8hours. The carbonated material is then allowed to age for 16 hoursduring which ageing" it is slowly agitated. The step of ageing with slowagitation assures uniformity in particle size and complete carbonationof the lime. Upon completion of the ageing process, the calciumcarbonate is recovered by filtering, drying and pulverizing.

The calcium carbonate prepared in this way meets the rigid requirementsas to fineness and uniformity of particle size for the highest grades ofprecipitated calcium carbonate. It is a substantially chemically purecalcium carbonate in that the insoluble impurities of the original rock,that are retained in processes that make precipitated calcium carbonatedirectly from limestones, are separated from it during the intermediatelime solution step of the magnesia separation process.

As a further modification of the original process set forth in myc'opending application, Serial No; 133,286, the following additions andchanges may be made therein.

Return solids and a fiocculant may be added to the material passing fromthe dissolvers 5 to the separators 6, thesolids being taken from theunderfiow of the separators (thickeners) i.

Again, the material passing from the primary carbonator I to the primarythickener 8 may have introduced into the same return solids as a part ofthe underflow from the primary thickener 8 and a fiocculant.Furthermore, in the line passing from the carbonator I? to the secondarythickener l5. one may introduce return solids taken for instance fromthe underfiow of said thickener. The process water will be dischargeddirectly into the line leading to the classifier 3. instead of beingprimarily admixed with the hot water.

These various changes or additions are indicated by dotted lines on flowsheet No. 1.

What is claimed is:

1. The continuous process of treating limestone having calcium andmagnesium components, comprising calcining said limestone to softburnedlime, slaking the soft-burned lime while hot with an excess of hot waterto form a milk of lime of finely divided particles and separating theparticles inexcess of 100 mesh size, dispersing the remaining particlesunder 100 mesh size by agitation, subjecting said higly dispersed andfinely divided-particles of magnesia-containing lime hydroxide to thedissolving action of water having a pH within the range of approximately9.6 to 9.8, said water being present in the ratio of 1000 to 1250 partsto 1 part of CaO dissolved therein with violent agitation to produce adilute solution containing dissolved therein substantially all theresulting calcium hydroxide and with the relatively insoluble magnesiumhydroxide and the remaining unsubstantial amount of undissolved calciumhydroxide held in suspension, separating the undissolvedmagnesia-containing particles from the water containing the dissolvedlime and recovering them, treating the water containing the lime insolution with CO2 to reduce the alkalinity of the solution to about a p3equal to 9.6 to 9.8, whereby a. finely divided calcium carbonate isprecipitated, separating the calcium carbonate from the water with a pHequal to 9.6 to 9.8 and recovering it, and reusing such water todissolve lime from the highly dispersed and finely divided suspension oimagnesiacontaining lime hydroxide particles.

2. The continuous process of treating limestone having calcium andmagnesium components comprising calcining said limestone to produce asoft-burned CaO component and carbon dioxide, slaking the calcinedmaterial while hot with an excess of hot water to form a milk of lime,and reducing the solid particles present in said milk of lime to afinely divided and dispersed state by agitation, whereby the calciumcontent is made more completely amenable to solution, subjecting saiddispersed particles to the action oi an aqueous dissolving agent withviolent agitation to produce a dilute solution containing dissolvedthereinsubstantially all the resulting calcium hydroxide and with therelatively insoluble magnesium hydroxide and the remaining unsubstantialamount of undissolved calcium hydroxide held in suspension, thickeningsaid dilute solution to separate the major portion of the undissolvedcomponents from the dilute solution of calcium hydroxide, carbonatingthe relatively clear solution of calcium hydroxide to convert a portionof the calcium content thereof into a product consisting primarily ofextremely fine calcium carbonate, and separating said calcium carbonatefrom the solution containing some unconverted calcium hydroxide, therebyrecovering separately the dnsoluble magnesium hydroxide and theprecipitated calcium carbonate.

3. The continuous process of treating limestone having magnesium andcalcium components comprising calcining said limestone at a temperatureadapted to produce soft-burned CaO, slaking the calcined limestone whileit is still hot with water having a temperature above 50' 0., the amountof water being sufllciently in excess of the chemical requirements ofthe hydrates to form a milk of lime, and reducing the solid particlespresent in said milk of lime to a finely divided and dispersed state byagitation, whereby the calcium content is made more completely amenableto solution, subjecting said dispersed particles to the actionof waterwith violent agitation to produce a dilute solution containing dissolvedtherein substantially all the resulting calcium hydroxide and with therelatively insoluble magnesium hydroxide and the remaining unsubstantialamount K of undissolved calcium hydroxide in suspension, thickening theso-treated product to remove the major portion of the undissolvedmagnesium and calcium hydroxides, carbonating the relatively clearsolution until the pH of the solution is approximately 11.5, whereby aportion of the dissolved lime content oithe solution is converted intocalcium carbonate, thickening the carbonat edsolution to produce aprimarysludge consisting'principally of calcium carbonate containingsome calcium hydroxide, and a separate secondary solution containingunconverted calcium hydroxide, carbcnating the primary sludge to convertthe contained calcium hydroxide into a carbonate, carbonating saidsecondary'solution, to

connect the calcium hydroxide dissolved in it into calcium carbonate,and recovering separately the insoluble magnesium hydroxide and theprecipitated carbonate. 1

4. The continuous process oi! treating limestone having calcium andmagnesium components comprising calcining said limestone to produce asoft-bumed CaO component and-carbon dioxide, slaking the calcinedmaterial with an excess of hot water to form a milk of lime, andreducing the solid particles present in said milk of lime to a finelydivided and dispersed state, whereby the calcium content is made morecompletely amenable to solution, subjecting said dispersed particles tothe action ofwater with violent agitation to produce a dilute solutioncontaining dissolved therein substantially all the resulting calciumhydroxide and with the relatively insoluble magnesium hydroxide and theremaining unsubstantial amount of undissolved calcium hydroxide insuspension, thickening said dilute solution to remove the major portionof the undissolved magnesium and calcium hydroxide and to form arelatively clear solution of- I calcium hydroxide, subjecting said clearsolution to a primary carbonation by passing therein CO: gas produced inthe calcining step until the pH of the solution is approximately 11.5 tothereby convert a portion or the dissolved lime content into calciumcarbonate, separating said calcium carbonate from the solutioncontaining unconverted calcium hydroxide, subjecting said lattersolution to a secondary carbonation step to convert the calciumhydroxide dissolved in it into calcium carbonate, and recoveringseparately the insoluble magnesium hydroxide and the precipitatedcalcium carbonate. g

V 5. The continuous process of treating limestone having magnesium andcalcium components comprising calcining said material at a temperatureadapted to produce soft-burned CaO, slaking the calcined limestone whileit is still hot with water having a temperature above,50C.,theamouritoi'waterbeingsumcientlyin excess of the chemicalrequirements of the hydrates to form a milk of lime, and reducing thesolid particles present in said milk of lime to a finely divided anddispersed state, whereby the calcium content is made more completelyamenable to solution, subjecting said dispersed parlcies to the actionof water with violent agitaof the dissolved lime content into calciumcarbonate, separating said calcium carbonate from the solutioncontaining unconverted calcium hydroxide, subjecting said lattersolution to a secondary carbonation step until the pH oi' the solutionis within the range of 9.6 to 9.8 to convert the calcium hydroxidedissolved in it into calcium carbonate and recovering separately the inasoluble magnesium hydroxide and the precipitated calcium carbonate.

6. The continuous process of treating limestone having magnesiumandcalcium components comprising calcining said limestone at a temperatureadapted to produce a soft-burned CaO component, slaking the calcinedlimestone while it is still hot'with water having a temperatime above 50C., the amount of water being sufliciently in excess of the chemicalrequirements of the hydrates to form a milk of lime,'

classifying the milk of lime to remove large or heavy aggregates,agitating the milk of lime to divide and disperse the particles ofcalcium hydroxide and magnesium hydroxide, adding more water andsubjecting the resulting dilute milk of lime to violent agitation toproduce a dilute solution containingsubstantially all the resultingcalcium hydroxide with the relatively insoluble magnesium hydroxide andthe remaining unsubstantial amount of undissolved calcium hydroxide heldin suspension, thickening the resulting solution containing dissolvedcalcium hydroxide and undissolved calcium and magnesium hydroxides insuspension, separating and recovering a clear solution of calciumhydroxide and magnesia'sludge, treating the clear solution of calciumhydroxide with 00: gas to convert the calcium hydroxide tocalciumcarbonate, and recovering the calcium carbonate from thesolution.

7. The continuous process comprising classitying an aqueous suspension01' calcium andmagnesium hydroxides to remove unusable materials,subjecting the so-classified suspension to agitation to reduce theparticles present in said suspension to a finely divided and dispersedstate,

whereby the calcium content is more completely amenable to solution,subjecting saidiinely dispersed particles to intimate contact with waterby means of violent agitation to produce a dilute solution containingdissolved calcium hydroxide and undissolved calcium and magnesium hy- Idroxides in? -'-isuspension, separately" recovering from said dilutesolution the major portion oi the undissolved magnesium and calciumhydroxides and a relatively clear solution containing dissolved calciumhydroxide, subjecting said relatively clear solution to treatment withCO: to convert a controllable portion of the calcium content to calciumcarbonate, separating the resulting calcium carbonate from the solution,treating the remaining solution with CO: to convert the remainingcalcium hydroxide of the solution to calcium carbonate, and separatelyrecovering the resulting precipitated calcium carbonate.

8. The continuous process comprising classiiying an aqueous suspensionof calcium and magnesium hydroxides to remove hydroxide particles largerthan 100 mesh and particles heavier than the hydroxides overflowed,subjecting the so-ciassified suspension to agitation to reduce theparticles present in said suspension to a finely divided and dispersedstate, whereby the calcium content is more completely amenable tosolution, subjecting said finely dispersed particles to intimate contactwith water by means of violent agitation to produce a dilute solutioncontaining dissolved therein substantially all the resulting calciumhydroxide and with the relatively insoluble magnesium hydroxide and theremaining unsubstantial amount of undissolved calcium hydroxide held insuspension, separately recovering from said dilute solution the majorportion of the undissolved magnesium and calcium hydroxides and arelatively clear solution containing dissolved calcium hydroxide,subjecting said relatively clear solution to treatment with CO: toconvert a controllable portion of the dissolved calcium hydroxide tocalcium carbonate, separating the resulting calcium carbonate, treatingthe remaining solution again with CO: to convert the remaining dissolvedcalcium hydroxide in the solution to calcium carbonate, and separatelyrecovering the resulting precipitated calcium carbonates.

9. In the continuous'pryaess of treating an aqueous solution containingdissolved calcium hydroxide, the steps of carbonating the solution untilthe pH thereof is approximately 11.5 where- 7 by a portion of thedissolved calcium hydroxide content is converted into calcium carbonate,consisting primarily of finely divided material, stopping thecarbonation at such point, and removing the carbonate thus formed.

10. In the continuous process of recovering lime from limestone havingcalcium and magnesium components, the steps of calcining the limestoneat a temperature adapted to produce a soft-burned CaO component, slakingthe calcined limestone while it retains a portion oi. its internal heat,but insufiicicnt to produce a great amount of steam, whereby thesolubility oi the calcium hydroxide in water is increased, subjectingthe slaked lime to the action of water having a pH of a range ofapproximately 9.6 to 9.8 with violent agitation to produce a dilutesolution of substantially all the calcium hydroxide with the relativelyinsoluble magnesium hydroxide and the remaining unsubstantial amount ofundissolved calcium hydroxide held in suspension, removing the insolublemagnesium hydroxide and undissolved calcium hydroxide from the dilutesolution, carbonating the remaining solution containing dissolvedcalcium hydroxide until the pH thereof is approximately 11.5, whereby aportion of the dissolved calcium hydroxide is converted into calciumcarbonate consisting primarily of finely divided material, stopping-thecarbonation at such point. and removing the carbonate thus formed. I

11. In the continuous process of recovering lime from limestone havingcalcium and magnesium components, the steps of calcining the limestoneat a temperature adapted to produce a soft-burned CaO component, slakingthe calcined limestone while it retains a portion of its internal heat.but insuflicient to produce a great amount of steam, whereby thesolubility of the calcium hydroxide in water is increased, subjectingthe slaked lime to the action of water having a pH within the range ofapproximately 9.6 to 9.8 with violent agitation to produce a dilutesolution of substantially all the calcium hydroxide with the relativelyinsoluble magnesium hydroxide and the remaining unsubstantial amount ofundissolved calcium hydroxide held in suspension, the water beingpresent in the ratio of 1000 to 1250 parts to 1 part of calcium oxide insolution, removing the insoluble magnesium hydroxide and undissolvedcalcium hydroxide from the dilute solution, carbonating the remainingsolution containing dissolved calcium hydroxide until the pH thereof isapproximately 11.5, whereby a portion of the dissolved calcium hydroxideis converted into calcium carbonate consisting primarily of finelydivided material,

, stopping the carbonation at such point, and

removing the carbonate thusiormed.

12. The continuous process of treating limestone having calcium' andmagnesium components comprising calcining said limestone to soft burnthe CaO component, slaking the calcined limestone while still hot withan excess of hot water to form a milk of. lime, and reducing the solidparticles present in said milk of lime to a finely divided and dispersedstate, whereby the calcium content is made more completely amenable tosolution, subjecting said dispersed particles to the action of waterhaving a pH within the range of approximately 9.6 to'9.8 with violentagitation, said water being present in the ratio of 1000 to 1250 partsto 1 part of CaO in solution to thereby produce a dilute solutioncontaining dissolved therein substantially all the resulting calciumhydroxide and with the relatively insoluble magnesium hydroxide and theremaining undissolved calcium hydroxide held in suspension, recoveringsaid dissolved and undissolved calcium hydroxide and the insolublemagnesium hydroxide from the dilute solution, producing a productcontaining not less than 90% MgO on a calcined basis, carbonating thedilute solution from which the particles in suspension have been removedto convert the dissolved calcium hydroxide content thereof into aproduct consisting primarily oI finely divided calcium carbonate,separating said calcium carbonate from the solution, and using theso-exhausted dilute solution to dissolve lime from the highly dispersed,finely divided suspension of magnesiacontaining lime hydroxideparticles.

13. The continuous process of treating limestone having calcium andmagnesium components comprising calcining said limestone to soft burnthe CaO component, slaking the calcined limestone while still hot withan excess of hot water to form a milk of lime, and reducing the solidparticles present in said milk of lime to a finely divided and dispersedstate, whereby the calcium content is made more completely amenable tosolution, subjecting said suspension of dispersed particles to theaction of water having a pH within the range of approximately 9.6 to9.8, said water being present in the ratio of 1000 to 1250 parts to 1part of CaO in solution while 76 area-94o subjecting said suspension toviolent agitation for a period of the order of 2 to 8 minutes, tothereby produce a dilute solution containing dissolved thereinsubstantially all the resulting calcium hydroxide and with therelatively insoluble magnesium hydroxide and the remaining undissolvedcalcium hydroxide held in suspension, recovering said dissolved andundissolved calcium hydroxides and the insoluble magnesium hydroxidefrom the dilute solution, carbonating the dilute solution from which theparticles in suspension have been removed to convert the dissolvedcalcium hydroxide content thereof into a product consisting primarily offinely divided calcium carbonate, separating said calcium carbonate fromthe solution, and reusing the so-exhausted dilute solution to dissolvelime from the highly dispersed, finely divided suspension ofmagnesiacontaining lime hydroxide particles.

14. In the method of dissolving lime from a finely divided and highlydispersed suspension of magnesia-containing lime hydroxide particles bymixing the suspension with 1000 to- 1250 parts of water to 1 part oflime in solution, the step of greatly reducing the solution time byagitatin the mixture at more than the herein described normal agitation.

15. In the method of producing a magnesia product containing not lessthan 90% MgO from a finely divided and highly dispersed suspension ofmagnesia-containing lime hydroxide particles by dissolving lime throughthe addition of 1000 to 1250 parts of water to 1 part of lime insolution, the step of reducing the solution time to the order ofapproximately 2 to 8 minutes by agitating the solution at about fourtimes the herein described normal agitation.

16. In the continuous process of separating lime and magnesia frommagnesia-containing limestones based on the solubility of lime hydroxidein water as set forth in claim 6, the step oi? eflecting the rapidseparation of the less soluble magnesia-containing residual particlesfrom the dissolved lime by the addition of both aflocculating agent andpreviously settled solids.

1'7. In the continuous process of separating lime and magnesia frommagnesia-containing limestones based on the solubility of lime hydroxidein water as set forth in claim 6, the step of accelerating the settlingrate of the less soluble magnesia-containing particles through theaddition of one pound oi flocculating agent in the form; of starch totwelve thousand gallons of water in the feed to a thickener and fiveparts by weight of previously settled .solids to one part by weight ofsolids suspended in the dissolver discharge flowing to the thickener,whereby the size of the settling apparatus required may be reduced toone-fifth of the size required with the natural settling rate.

18. In the continuous method of separating precipitated calciumcarbonate from the aqueous solution of lime obtained in the process ofseparating lime from magnesia-containing limestones based on thesolubility of ,lime hydroxide in water as set forth in claim 6, the stepof accelerating the separation of precipitated calcium carbonate fromthe water by the addition of both a flocculating agent and previouslysettled calcium carbonate.

19. In the continuous method of separating precipitated calciumcarbonate from the aqueous solution of lime obtained in the process ofseparating lime from magnesia-containing limestones based on thesolubility of lime hydroxide in water as set forth in claim 6, the stepof accelerating the setting rate of calcium carbonate from the water,which consists in adding approximately one pound of flocculating agentin the form of starch to approximately twelve thousand gallons .of waterand adding five parts by weight of previously settled calcium carbonateto bne part by weight of calcium carbonate suspended in the carbonatordischarge flowing to the thickener, whereby the sire of the settlingapparatus required may be reduced to one-twelfth of the size requiredwith the natural settling rate.

20. In the continuous process of separating lime and magnesia frommagnesia-containing limestones based on the solubility oflime hydroxidein water as set forth in claim 6, the step of increasing the rate ofseparation of the less soluble magnesia-containing residual particlesfrom the dissolved lime wherein a flocculating agent is used, which stepconsists in adding previously settled solids to the solution.

21. In the continuous process of separating lime and magnesia frommagnesiacontainlng limestones based on the solubility of lime hydroxidein wateras set forth in claim' 6, the steps of accelerating the settlingrate of the less. solu uble magnesia-containing particles through theaddition of one pound of flocculating agent in the form of starch totwelve thousand gallons of water in the feed to a thickener, and offurther accelerating the settling rate by the addition of five parts byweight of previously settled solids to one part by weight of solidssuspended in the dissolver .discharge flowing to the thickener, wherebythe size of the settling apparatus required may be reduced to one-halfof the size required'when the flocculating agent alone used.

22. In the continuous method of separating precipitated calciumcarbonate from the aqueous solution of lime obtained in the process ofseparating lime from magnesia-containing limestones based on thesolubility of lime hydroxide in water as set forth in claim 6, the stepof increasing the rate of separation of the precipitated calciumcarbonate from the water wherein a flocculating agent is used, whichstep consists in adding to the solution previously settled calciumcarbonate.

23. In the continuous method of separating solution of lime obtained inthe process of separating lime from magnesia-containing limestones basedon the solubility of lime hydroxide in water as set forth in claim 6,the steps of acclerating the settling rate of the calcium carbonate bythe addition of one pound of flocculating agent in the form of starch totwelve thousand'gallons of water in the feed to a thickener, and offurther accelerating the settling rate by the addition of five parts byweight of previously settled calcium carbonate to one part by weight ofcalcium carbonate suspended in the carbonator discharge flowing to thethickener, whereby the sire of the settling apparatus required may bereduced to one-half of the size required when the flocculating agentalone is used.

24. In the continuous process of separating lime and magnesia frommagnesia-containing limestones based on the solubility of lime hydroxidein water as set forth in claim 6, the step of producing achemicallypure, finely divided, high grade precipitated calcium carbonate from theordinary precipitated calcium carbonate res covered in the separationprocess by recalcining,

recarbonating the precipitated calcium carbonate recovered in theprocess.

25. In the continuous process of separating lime and magnesia frommagnesia-containing limestones based on the solubility of lime hydroxidein water as set forth in claim 6. the steps which consist of calciningthe recovered. calcium carbonate to lime, slaking the lime with fourparts by weight of water, cooling the hydrate to about 40 C. and addingan equal amount of water, classifying the milk of lime to under 100mesh, carbonating the classified milk 01' lime until the pH of theliquid is less than 8 at a'temperature under 60 C. while subjecting itto normal agitation, and ageing the carbonated material for a period ofthe order of 16 hours while subjecting it to less than normal agitation,to produce a chemically pure, finely divided, high rade precipitatedcalcium carbonate.

26. The continuous process comprising classilying an aqueous suspensionof calcium and magnesium hydroxides to remove unusable materials,subjecting the so-classified suspension to agitation to reduce theparticles present in said suspension to a finely divided and dispersedstate, whereby the calcium content is more completely amenable tosolution, subjecting said finely dispersed particles to intimate contactwith water by means 01' violent agitation to produce a dilute solutioncontaining dissolved therein substantially all the resulting calciumhydroxide and with the relatively insoluble magnesium hydroxide and theremaining unsubstantial amount 01' undissoived calcium hydroxide insuspension, separately recovering from said dilute solution the majorportion of the undissoived magnesium and calcium hydroxides and arelatively clear solution containing dissolved calcium hydroxide,subjecting said relatively clear solution to treatment with CO; toconvert a controllable portion of the calcium content to calciumcarbonate, separating the resulting calcium carbonate from the solution,treating the remaining solution with C: to convert the remaining calciumhydroxide of the solution to calcium carbonate, and separatelyrecovering the resulting precipitated calcium carbonate.

27. The continuous process comprising classifying an aqueous suspensionof calcium and magnesium hydroxides to remove unusable materials,subjecting the so-classified suspension to agitation to reduce theparticles present in said suspension to a finely divided and dispersedstate, whereby the calcium content is more completely amenable tosolution, subjecting said finely disprsed particles to intimate contactwith additional water by means of violent agitation to from limestonehaving calcium and magnesium components comprising calcining thelimestone at a temperature adapted to produce a soft burned CaO,component, slaking the calcined limestone while still hot to produce amilk of lime, classifying the resultingaqueous suspension of calcium andmagnesium hydroxides to remove unusable materials, subjecting thesoclassified suspension to agitation to reduce the particles present insaid suspension to a finely divided and dispersed state, whereby thecalcium content is more completely amenable to solution, subjecting saidfinely dispersed particles to intimate contact with additional water bymeans of violent agitation to produce a dilute solution containing themajor portion oi-the calcium hydrox ide dissolved therein andundissolved calcium and magnesium hydroxides in suspension, separatelyrecovering from said dilute solution the major portion of theundissolved magnesium and calcium hydroxides and a relatively clearsolution containing dissolved calcium hydroxide, subjecting saidrelatively clear solution to treatment with CO: to convert acontrollable portion of the calcium content to calcium carbonate, andseparating the resulting calcium carbonate from the solution.

WTLHAM J. YOUNG.

CERTIFICATE OF CORRECTION. I Patent No. 2,195,919. I April 9, 191m.

WILLIAM J. 120mm. v

It is hereby certified, that error appears in the printed specificationof .the above numbered patent requiring correction as follows: Ifage 8,sec-- 0nd column, line 21, claim 1, for "higly" read --highly--; pagefirst column, line )2, claim 5, for "connect" read --conve rt--; line514555, claim 14., for. hydroxide read --hydroxides-.--;-8.nd secondcolumn, line [41 claim 6-; for the word "dilute" read -diluted--; andthat the said .Letters Pat-.- ent should be read with this correctiontherein that the same may conform to the record of the case in thePatent Office.

Signed and sealed this 18th day of June, A. '1), 191m.

. Henry Van Arsdal'e,' (Seal) Acting Commissioner of Pat ents.

